Rechargeable batteries have been used for many years as portable sources of DC power. Commercially useful versions range from the well-known lead/acid batteries, which are commonly used to provide starting and ignition power for internal combustion engines, to the more modern nickel/cadmium (NICAD) batteries, which are currently used in power tools and household appliances, and to nickel/hydrogen (Ni/H.sub.2) batteries, which are used in spacecraft.
All the above batteries, which contain aqueous electrolytes, have acceptable cycle lives but fairly limited energy densities. Specific energy is defined as the battery power in watts (W) multiplied by its life in hours (h) divided by its weight in kilograms (kg). Thirty Wh/kg is typical for lead/acid and NICAD batteries and fifty Wh/kg is typical for Ni/H.sub.2 batteries. However, while lead/acid and NICAD batteries enjoy wide commercial acceptance, they contain toxic materials. Lead/acid batteries usually are reclaimed, but NICAD batteries are discarded, leading to environmental damage.
Secondary batteries with non-aqueous electrolytes provide much higher specific energy, offering the potential for much reduced battery size or increased battery power. FIG. 1 is an illustration of the power and energy density ranges of several varieties of batteries. The area labeled 10 represents the range for Ni/H.sub.2 batteries; 12 represents the range for sodium/sulfur batteries; 14 represents the range for sodium/metal chloride batteries; 16 represents the range for NICAD batteries; 18 represents the range for lithium/titanium disulfide batteries; and 20 represents the range for lithium/metal oxide and lithium/polyethylene batteries.
It can be seen from FIG. 1 that batteries based on lithium (Li) and sodium (Na), i.e. which have one electrode made of Li, Na, or a Li alloy, can provide from about 50 to 200 Wh/kg. Such batteries utilize organic, inorganic or molten salt electrolytes. Li batteries seem to be the most practical for commercial applications requiring low weight and volume.
However, to date, usage of Li-based secondary batteries with organic electrolytes has been fairly limited. This is mainly due to their poor cycle life and relatively low electrolyte conductivity. For example a currently available lithium/molybdenum (Li/Mo) battery, has an energy density of about 60 Wh/kg and a cycle life of around 200 charge/discharge cycles.
The lithium or lithium alloy electrode, in lithium secondary batteries, is called the anode in conventional battery terminology. Since lithium is a soft metal, it is usually press bonded to a supporting grid made of a more rigid metal, such as nickel. Since lithium readily reacts with moisture in the atmosphere, special dehumidified facilities are required for fabrication of anodes and batteries.
The other electrode, which is called the cathode in conventional battery terminology, is usually made of certain metal chalcogenides or oxides supported by a polymer matrix. Typical of these metal compounds are TiS.sub.2, NbSe.sub.3, V.sub.6 O.sub.13, V.sub.2 O.sub.5, MoS.sub.2, MoS.sub.3, CoO.sub.2, and CrO.sub.2. Ethylene propylene diene terpolymer is commonly used as the supporting polymer matrix.
Electrolytes used in such batteries are generally composed of some sort of lithium salt dissolved in a non-aqueous solvent. Many different salts have been used including LiCIO.sub.4, LiBF.sub.4, LiAsF.sub.6, and LiPF.sub.6. Solvents have included propylene carbonate (PC), 1,3-dioxlane (DOL), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), 3-methylsulfolane (3-MeS), and methylformate.
One further component of all lithium-based secondary batteries is a separator, which is permeable to the electrolyte and which simply separates the anode and cathode to prevent shorting.
However, it has been found that all state-of-the-art electrolytes have a fairly low conductivity and react with the lithium anode to some degree. It is this reaction that reduces the useful cycle life of lithium secondary batteries. Some investigators have proposed complex ways of fabricating the anode or coating the anode with a protective coating to reduce this effect.
If an electrolyte could be found that had high conductivity but which were less corrosive to the lithium anode, it would satisfy a long felt need in the field of secondary battery technology since it would extend the useful life of the battery considerably. This would permit widespread use of lithium secondary batteries in such commercial and industrial applications as portable tools, appliances, cellular phones, computers, and in such space applications as the Mars Rover and the astronaut's backpack.